Apparatus and method for heating microfluidic volumes and moving fluids

ABSTRACT

A device and method for the temperature control, concentration, volume measurement and transport of microfluidic volumes are provided. The device includes one or more heating elements having a resistive material that varies with temperature. The heating elements are formed into a laminar body that may be formed into a variety of geometries and/or easily married to a second body including micro-well plates, micro-centrifuge tubes and microfluidic circuits.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to U.S. patent application Ser. No.10/765,536 entitled “Apparatus and method for heating microfluidicvolumes and moving fluids” filed on Jan. 27, 2004 which is incorporatedherein by reference in its entirety and which claims priority to U.S.Provisional Application Ser. No. 60/443,209, entitled “Method forheating microfluidic circuits and moving fluids”, filed on Jan. 27, 2003which is incorporated herein by reference in its entirety.

FIELD OF THE INVENTION

The invention relates generally to microfluidic devices and, inparticular, microfluidic devices comprised of one or more heatingelements having a resistance that varies with temperature.

BACKGROUND OF THE INVENTION

Manipulating fluidic reagents and assessing the results of reagentinteractions are central to chemical and biological science.Manipulations include mixing fluidic reagents, assaying productsresulting from such mixtures, and separation or purification of productsor reagents and the like. Assessing the results of reagent interactionscan include autoradiography, spectroscopy, microscopy, photography, massspectrometry, nuclear magnetic resonance and many other techniques forobserving and recording the results of mixing reagents. A singleexperiment can involve literally hundreds of fluidic manipulations,product separations, result recording processes and data compilation andintegration steps. The effects of mixing fluidic reagents are typicallyassessed by additional equipment relating to detection, visualization orrecording of an event to be assayed, such as spectrophotometers,autoradiographic equipment, microscopes, gel scanners, computers and thelike. Fluidic manipulations are performed using a wide variety oflaboratory equipment, including fluidic mixing devices, centrifugationequipment, molecule purification apparatus, chromatographic machinery,gel electrophoretic equipment and various fluid heating devices.

An example of where heating devices are important is the amplificationof nucleic acids which is central to the current field of molecularbiology. Library screening, cloning, forensic analysis, genetic diseasescreening and other increasingly powerful techniques rely on theamplification of extremely small amounts of nucleic acids. As thesetechniques are reduced to a smaller scale for individual samples, thenumber of different samples that can be processed automatically in oneassay expands dramatically. Microscale devices have evolved which canhave few to hundreds of fluidly connected channels, conduits, chambersand wells for handling mircofluidic volumes. New integrated approachesfor the handling and assaying of a large number of small samples areneeded.

In particular, new integrated approaches for the precise temperaturecontrol of microfluidic volumes are needed. For example, in thepolymerase chain reaction (PCR) for nucleic acid amplification, apurified DNA polymerase enzyme is used to replicate the sample DNA invitro. This system uses a set of at least two primers complementary toeach strand of the sample nucleic acid template. Initially, the samplenucleic acid is heated to cause denaturation to single strands, followedby annealing of the primers to the single strands, at a lowertemperature. The temperature is then adjusted to allow for extension ofthe primers by the polymerase along the template, thus replicating thestrands. Subsequent thermal cycles repeat the denaturing, annealing andextending steps, which results in an exponential accumulation ofreplicated nucleic acid products. The accuracy and reproducibility ofthe microfluidic analyses can be highly dependent on the temperature ofthe fluid volume. Robust heating devices that can accurately control thetemperature of microfluidic volumes are required.

The concentration of fluids is another field where heating devices comeinto play. In many chemical and biochemical analysis methods performedusing microfluidic devices, it is advantageous to concentrate an analyteas part of the analysis. For example, increased analyte concentrationgenerally leads to increased chemical reaction rates, increased rates ofmass transfer, and enhanced detectability.

One general problem which has not been solved for microfluidics andwhich the present invention presents is an integrated, reproducible, andinexpensive temperature control for heating, thermal cycling,concentration of fluids, volume measurement, sensing and fluidtransport. Prior art solutions in the form of “thermofoils” attempt tosolve part of this problem but they involve incorporating resistancetemperature detector devices or thermistors into the film and aretherefore quite expensive. It is an object of the present invention toprovide low-cost heating element that is affixable to a variety oftemperature-sensitive devices.

SUMMARY OF THE INVENTION

According to one aspect of the invention, a device comprising a laminarbody is provided. The laminar body includes a substrate having a firstsurface and a second surface. The laminar body includes at least oneheating element disposed on the first surface. The heating elementcomprises a conductive layer that is patterned into at least twoelectrodes in a spaced relation to each other. The heating element alsoincludes a resistive layer that includes a resistive material having aresistance that changes with temperature at a predetermined resistancetemperature coefficient. The resistive layer is disposed to permitcurrent to flow through the resistive material between the electrodes.The laminar body further includes at least one fluid-receiving locationthat corresponds to the location of the at least one heating element.The heating element is in thermal communication with the fluid-receivinglocation.

According to another aspect of the invention a method for concentratingand measuring microfluidic volumes is provided. The method includes thestep of providing a laminar body that includes a substrate with a firstsurface and a second surface. At least one heating element is disposedon the first surface of the substrate. The heating element comprises aconductive layer patterned into at least two electrodes in spacedrelation to each other and a resistive layer comprising a resistivematerial having a resistance that changes with temperature at apredetermined resistance temperature coefficient. The resistive layer isdisposed between the electrodes to permit current to flow through theresistive material between the electrodes to generate heat. The laminarbody also includes at least one fluid-receiving location thatcorresponds to the location of the at least one heating element. Theheating element is in thermal communication with the fluid-receivinglocation. The method further includes the steps of placing a volume offluid at the fluid-receiving location and providing an electronicscomponent having at least signal detection circuitry and controlcircuitry connected to the at least one heating element. The methodfurther includes the step of applying a voltage across at least oneheating element. Another step is obtaining at least one electricalinformation from at least one of the heating elements. The electricalinformation is a function of the variable resistance of the resistivematerial. The electrical information of at least one heating element ismonitored. At least one heating element is controlled based on theelectrical information that is calibrated to correspond to a known fluidvolume or temperature and the fluid volume or temperature is determined.

According to another aspect of the invention, a method for movingmicrofluids is provided. The method includes the step of providing alaminar body that comprises a substrate having a first surface and asecond surface and at least one heating element disposed on the firstsurface. The heating element includes a conductive layer and a resistivelayer. The conductive layer is patterned into at least two electrodesdisposed in spaced relation to each other. The resistive layer comprisesa resistive material having a resistance that changes with temperatureat a predetermined resistance temperature coefficient. The resistivematerial is disposed between the electrodes to permit current to flowthrough the resistive material between the electrodes. The methodincludes the step of providing at least a first receiving locationinterconnected to a second receiving location. The heating element islocated next to the first receiving location. The method includes thestep of placing a volume of fluid in the second receiving location andplacing a volume of gaseous fluid in the first receiving location. Themethod includes the step of heating the volume of gaseous fluid toexpand and exert pressure on the volume of fluid in the second receivinglocation to move the volume of fluid in the second receiving location.The method includes the step of moving the volume of fluid.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

The foregoing and other advantages of the invention will become apparentupon reading the following detailed description and upon reference tothe drawings in which:

FIG. 1 is a perspective view of a device depicting a substrate,resistive material and conductive material according to the invention;

FIG. 2 is a side elevational view along cross-section 2-2 of FIG. 1according to the invention;

FIG. 3 is a top view of circuit pattern of the conductive layeraccording to the invention;

FIG. 4 is a perspective view of a laminar element having cone-shapeddimples according to the invention;

FIG. 5 is a perspective view of a laminar element havingspherically-shaped dimples according to the invention;

FIG. 6 is an exploded view of a laminar element and a second bodyaccording to the invention;

FIG. 7 is a perspective view of a laminar element and a second bodyaccording to the invention;

FIG. 8 is a graph of electrical current and temperature behavior overtime of a device according to the invention;

FIG. 9 a is a side view of a reservoir with one or more heating elementsaccording to the invention;

FIG. 9 b is a side view of a reservoir with one or more heating elementsalong at least a portion of the reservoir surface according to theinvention;

FIG. 10 a is a top view of one embodiment of a laminar body having oneor more heating elements arranged in a concentric pattern according tothe invention;

FIG. 10 b is a perspective view of a drop of fluid on one embodiment ofa laminar body having one or more heating elements arranged in aconcentric pattern according to the invention; and

FIG. 11 is a top view of a drop of fluid on one embodiment of a laminarbody having one or more heating elements according to the invention.

While the present invention is susceptible to various modifications andalternate forms, specific variations have been shown by way of examplein the drawings and will be described herein. However, it should beunderstood that the invention is not limited to the particular formsdisclosed. Rather, the invention is to cover all modifications,equivalents, and alternatives falling within the spirit and scope of theinvention as defined by the appended claims.

DETAILED DESCRIPTION OF THE INVENTION

This application claims priority to U.S. patent application Ser. No.10/567,536 entitled “Apparatus and method for heating microfluidicvolumes and moving fluids” filed on Jan. 27, 2004 which is incorporatedherein by reference in its entirety and which claims priority to U.S.Provisional Application Ser. No. 60/433,209, entitled “Method forheating microfluidic circuits and moving fluids”, filed on Jan. 27,2003, which is incorporated herein by reference in its entirety.

A diagram of a device 10 according to the invention is shown in FIGS. 1and 2. The device 10 of the invention comprises a conductive layer 12, aresistive layer 14 and a substrate 16. Typically, the conductive layer12 which comprises a conductive material is formed on the substrate 16and the resistive layer 14 which comprises a resistive material isformed on the conductive layer 12 as shown in FIGS. 1 and 2.Alternatively, the resistive layer 14 is formed on the substrate 16 withthe conductive layer 12 formed on the resistive layer 14. In onevariation, a second conductive layer and a second resistive layer isprinted on the second side 18 of the substrate 16.

The substrate layer 16 is generally electrically non-conducting and madeof any polymeric material known in the art to be suitable for thepresent application. The substrate material is selected to be easilyformable from a planar geometry into a variety of shapes with littledamaging distortion or stretching of resistive and/or conductive layers.The substrate material is also selected to be suitable for resistive orconductive material adhesion such that the resistive and conductivematerials will not shed easily from printing, forming or flexure. Withrespect to adhesion, resistive and conductive materials are, in oneembodiment, printable inks that typically contain solvent and require tobe cured at high temperature. The curing process evaporates the solvent,but the substrate material should be substantially unaffected ordegraded by the high temperature for its purposes to be later formed andflexed for example. Generally, the substrate is between 0.002 inches and0.050 inches in thickness and flexible. A thinner substrate materialprovides greater heat transfer through the substrate when compared to athicker substrate material with the thickness being selected accordinglyfor the given application.

Examples of suitable substrate materials are thermoplastics includingpolyester such as polyethylene terephthalate (PET). One example of asuitable polyester film is provided by General Electric Plastics underthe trade name Valox PTX® 110 which is an optically clear and glossypolyester film with one side pretreated to promote ink adhesion. Anotherexample of a suitable substrate material includes polycarbonate such asBayer Corporations Macrofol product. Yet, another example of a suitablesubstrate material includes polybutylene terephthalate (PBT) such asthat provided by General Electric Plastics under the trade name GE ValoxFRI. Also suitable are PBT copolymers such as polycarbonate/polybutyleneterephthalate (PC/PBT) blends such as Bayfol provided by the BayerCorporation. PET copolymers such as polycarbonate/polyethyleneterephthalate (PC/PET) blends are also suitable such as Xylex™ providedby GE Plastics. Some materials are more suitable than others possessingcharacteristics more suitable for example for thermo-forming versuscuring inks and can be selected accordingly so as to provide the optimumcharacteristic for a given application.

The conductive layer 12 comprises a conductive material. The conductivematerial is generally a printable conductive ink. In one embodiment, theconductive material includes a polymer such as a polyester resin mixedwith conductive material such as silver flakes and a solvent. Themixture is kneaded into a paste suitable for use as an ink. However, anyconductive medium such as copper, gold, nickel, other metals and metalalloys is employable in a similar mixture or as a different non-inkmaterial. The conductive material is also suitable for withstandingthermoforming after application to the substrate. Examples of conductivematerials in the form of inks that are useful according to the inventioninclude Dupont 5000, 5025, 5028, 5096 and other equivalent commerciallyavailable conductive inks.

The conductive material is applied to the substrate in a variety oftechniques available in the art. For example, the conductive ink isscreen-printed using methods and techniques well known in the art;however, the conductive material can be applied by contact printing,gravure, etching, ink jet, photoresist technology, painting, spraying orother forms such as spread coating, casting, roller coating, spraycoating, sputtering, curtain-flow coating, brush coating and dipping.After being applied, the conductive material is allowed to dry or isbaked forming the conductive layer on the substrate in a thickness ofapproximately 2 to 20 microns.

The layers are formed in solid and/or intricately-patterned coatings toconfigure a variety of electrical circuits. Using these applicationmethods, a two-dimensional array of virtually any combination ofelectrical characteristics which mimic any discrete electronic devicesuch as resistors, capacitors, inductors, transistors and the like canbe created. In addition, through holes can be created in order tofacilitate routing of the circuitry. The circuits can be terminated in avariety of ways in order to facilitate the measurement of current,temperature, and power for example. Connections such as ZIF, surfacemount, crimped, pin and socket or others can be used.

The basic arrangement of the conductive material is shown in FIG. 1wherein at least two spaced-apart electrodes 20 and 22 are formed in theconductive layer as shown in FIGS. 1 and 2. The pattern of theelectrodes in FIG. 1 is a circular pattern in which the electrodes 20,22 are alternately interspaced in a concentric pattern. In oneembodiment, the diameter of the circular portion of the electrodes isapproximately 0.250 inches. The width of the conductive material in thepattern as measured parallel to the face of the substrate isapproximately 0.010 inches and the tracers of conductive material arespaced apart in gaps, as measured parallel to the face of the substrate,of approximately 0.015 inches in width. Also, the electrodes are shownpositioned and dimensioned that at all points, the distance betweenadjacent electrodes between which current passes, measured parallel tothe face of the substrate are substantially parallel. However, theinvention is not so limited and any circuit pattern is within the scopeof the present invention. In particular, the invention is not limited tothe shape, size and widths of the material and spacing but can betailored for exhibiting a particular electrical behavior for a givenapplication. In another embodiment, variation of the distance betweenelectrodes need not be uniform and can be positioned and dimensioned ifnon-uniform conduction across electrodes is desired. The circulargeometry allows minimum distortion of tracer spacing after the substrateis operation formed to produce dimples. Also, the circular geometry mayinduce concentric concentration of samples as will be disclosedhereinbelow.

In the embodiment shown in FIG. 1, the conductive layer also includestwo contacts 24, 26 that are formed with the electrodes, 20 and 22,respectively for coupling the electrodes to a source of electricalcurrent. The contacts 24, 26 are generally straight strips of conductivematerial.

Another pattern in which the conductive material is printed on thesubstrate is shown in FIG. 3. FIG. 3 shows a polygonal pattern with twospaced-apart electrodes 28, 30. Each of the electrodes 28, 30 has acontact 32, 34, respectively. The contacts 32, 34 are substantiallyparallel. Interdigitated comb-like “fingers” 36, 38 associated withelectrodes 28, 30, respectively, extend between the contacts. The“fingers” alternate and are spaced-apart to form gaps between adjacentfingers of opposite electrodes. As shown, the fingers 36 of oneelectrode 28 that are connected to one polarity of electrical currentinterdigitate in a comb-like pattern with the fingers 38 of the secondelectrode 30 formed in a comb-like pattern and connected to an oppositepolarity of electrical current.

The size of the block of interdigitated fingers 36, 38 is 0.150 by 0.150inches, 0.250 by 0.250 inches, 0.340 by 0.340 inches or 0.680 by 0.680inches; however, the block can be any shape and size. The width of eachfinger is approximately 0.005 to 0.025 inches. The width of the gapsbetween adjacent fingers, as measured parallel to the substrate surface,is approximately 0.005 to 0.025 inches. The spacing betweeninterdigitated traces determines in part the heating characteristics asresistive material will be located between the fingers bridging theelectrical current. Heater function can be tuned by selecting tracespacing and geometry and, of course, the thickness of the resistancematerial. Any shape, dimension, or pattern is selectable according tothe demands of the application and any number of heating elements can beformed.

The resistive layer 14 of FIGS. 1 and 2 comprises a resistance material.In one embodiment, the resistance of the resistance material varies withtemperature. In on variation, the resistance material exhibits apositive temperature coefficient (PTC) of resistance behavior and istherefore, a PTC material. The resistance material is an ink suitablefor printing. A PTC material increases in resistivity with an increasein temperature, and typically exhibits a sharp change in resistivity ata certain temperature, T_(s), known as the switching temperature. In oneembodiment of the invention, the PTC composition has a preselectedswitching temperature

Generally, manufacturing a PTC material involves dissolving acrystalline polymer in a suitable solvent to form a single phase polymersolution, and dispersing electrically conductive particulate material inthe polymer solution. Any suitable crystalline polymer is useful and mayinclude crystalline polymers solvatable at ambient temperatures.Examples of polymers include crystalline polyurethane and choroprenepolymers. Examples of solvents include acetone, methyl ethyl ketone,cyclohexanone, carbitol acetate, and solvent blends. The selection ofthe solvent and polymer will depend in part upon several factors such asmethod of application of the composition, cost, and evaporation rate.Many different kinds of conductive particles are suitable for forming aPTC composition, including metal powders such as silver, gold, nickel,powdered graphite, metallic carbides, high-structured carbon black andlow-structured carbon black. The conductive particulate material iswell-dispersed into the formulation to give the compositionsubstantially uniform electrical properties. The resistive material canbe any resistance material held in binder and which preferably can beapplied by a printing technique. Also, the composition of resistancematerials can also contain non-conductive fillers, includingarc-suppression agents, antioxidants, and other auxiliary agents. Inksof the general type used are available from DuPont such as DuPont 7082,7102, 7271, 7278, 7282 and 7285. Other equivalent commercially availablePTC inks are also useful. Many such compositions are described invarious patents and publications listed or disclosed herewith and areincorporated by reference herein.

The invention also includes resistance materials that do not exhibit PTCbehavior. The specific resistance of the particle-binder mixture may bevaried by changing the amount and kind of electrically conductiveparticles used. In addition to the amount of carbon black, thecrystallinity of the polymer is an important factor to be consideredwith respect to its influence on the electrical characteristics of thecomposition as will be appreciated by one skilled in the art. The ratioof particulate matter to the type of polymeric binder can vary dependingon the range of values of the resistance factor used in the device.Also, the amount of solvent affects resistance factor and heaterfunction. The amount of solvent remaining in the composition can betuned by partially curing the resistance material to target a desiredresistance. The composition of the resistive material of the presentinvention is not restricted and can be selected from a wide range tomeet particular physical and electrical performance requirements of thedevice.

Typically, the resistive material is applied on the conductive layer byscreen printing or using any of the other methods described above withrespect to the conductive layer. The conductive layer can be located oneither side of the resistive coating as long as they are in contact. InFIG. 1, the resistive material is applied in a circular pattern betweenthe electrodes and in contact therewith such that a current flowingthrough the electrodes passes through a substantial portion of theresistive material layer at a temperature at which the resistivematerial is conductive. In one embodiment, the resistive layer 14 is notapplied onto the contacts 24, 26 as shown in FIG. 1. In FIG. 3, theresistive material is applied in a polygonal pattern between theelectrodes and in contact therewith such that a current flowing throughthe electrodes passes through a substantial portion of the resistivematerial layer at a temperature at which the resistive material isconductive. In one embodiment, the resistive layer is not applied ontothat portion of the electrodes 28, 30 that form contacts 32, 34 as shownin FIG. 3.

After the resistive layer is printed, it is then baked at a prescribedtemperature for a prescribed time to form a resistive layer. Theresistive layer is approximately 2.0-20.0 microns thick as measuredperpendicular to the face of the substrate. Repetitive screen printingof resistive inks can be used to deposit thicker layers having reducedresistances. Generally, the thickness is selected so that the resistancerecovering characteristics from a high temperature is not degraded andso that the solvent is completely evaporated in order to avoid anyresulting instability in resistance characteristics. The thickness ofthe resistive layer affects heater function such that a thicker layergenerally results in a lower resistance and more heat/power. The abilityto tailor the resistance and hence, temperature profile of the resistiveheaters using choice of ink formulation and printing/dimensioning of thecircuit advantageously provides control of the final electrical andthermal properties of the device when in operation.

In one embodiment, a contact layer (not shown) of material with heatresistance and good thermal conductivity is applied to sandwich theresistive and conductive layers between the contact layer and thesubstrate either before or after the second forming operation. Inanother embodiment, an additional second contact layer is applied tosandwich the resistive, conductive and substrate layers between thefirst and second contact layers. The contact layer serves to insulatethe electrical circuit and to prevent oxidation and water or any otherelectrolyte from contacting and bridging the electrodes and thereforeavoids the possibility of short circuits between the electrodes andconsequent sparking and burning of the laminar element. Contact layersadvantageously comprise dielectric inks such as Dupot 5018A. Insulationcan also be achieved using pressure sensitive transfer adhesives orthermoplastic bonding films. In one embodiment, the contact layer is amicrofluidic circuit comprising, chambers, conduits, channels and thelike for handling and processing microfluids wherein temperatures areprecisely controlled for a variety of applications.

The resulting laminar heating device 10 is very thin, approximately, 135to 300 microns thick. This feature enables the device to be massproduced and simply tailored to suit the needs of the user. The presentinvention provides a flexible, formable, and easily produced electricalheating device. It can be shaped and cut to accommodate the needs of theuser. The laminar element can be formed into a non-planar element by anumber of processing including insert molding, converting methods thatoften involve sheets, films, or tapes that can be handled in acontinuous or semi-continuous manner. The continuous tapes can result inthe production of thousands to millions of discrete devices usingconverting processes such as cutting, lamination, printing, anddimpling. The raw materials that are fed into the converting process areoften flat. Structures such as wells, channels and chambers can beformed using multiple layers of flat films or by distorting the flatfilm for example by dimpling and vacuum forming. Although printing thecircuit prior to forming has been described, the invention is no solimited and printing or forming circuits can be performed subsequent toforming reservoirs and other shapes in the laminar device. In somecases, printing onto a shaped portion of the laminate after the shapehas been formed is advantageous. This can be done, for example, bycontact printing directly onto the shape.

Referring now to FIG. 4, the laminar element 40 is formed with aplurality of dimples 42. In one embodiment, the dimples 42 arecone-shaped and thermoformed in alignment with or next to thecircular-shaped pattern for electrodes as depicted in FIG. 1. However,the invention is not so limited and dimples of any shape and depth canbe formed with any pattern of conductive and resistive material. Duringthe forming process there may be some deformation of the conductive andresistive layers; however, this distortion is minimized by formingdimple geometries that are more compatible and result in less distortionwhen formed with a particular circuit pattern. The cone-shaped or“drill-bit” dimple is approximately 0.250 inches in diameter andapproximately 0.100 inches in depth as measured from surface of thesubstrate to the tip of the dimple. The cone-shaped or “drill-bit”dimple is aligned with the center of the circular-patterned electrodesthat have a diameter of 0.250 inches. In another variation, thecone-shaped dimple is between approximately 0.187 to 0.300 inches indiameter and approximately 0.05 to 0.30 inches in depth.

In another embodiment shown in FIG. 5, the dimples 44 of the laminarbody 46 have a spherical geometry. The spherically-shaped dimples arethermoformed in alignment with or next to the circular-shaped circuitpattern of the type depicted in FIG. 1. The spherically-shaped dimple isapproximately 0.250 inches in diameter and approximately 0.100 inches indepth as measured from surface of the substrate to the tip of the dimpleand formed in alignment with the center of the circular patternedelectrodes having a diameter of 0.250 inches. In another variation, thespherically-shaped dimple is between approximately 0.187 to 0.300 inchesin diameter and approximately 0.05 to 0.30 inches in depth.

The dimples serve as receptacles for receiving fluid and locating fluidin thermal communication with and next to the circuit. Fluid is placeddirectly into a dimple. In one embodiment, the interior of the dimple isformed on the surface opposite to the surface on which the circuit isprinted. In another embodiment, the interior of the dimple is formed onthe same surface as the surface on which the circuit is printed. In thelater case, a contact layer or second body is overlaid on the laminarbody to prevent shorting out the circuit. In another embodiment, theplurality of dimples is formed having a pitch of approximately 9 mmbetween adjacent dimples.

In one variation, the laminar element 48 with formed dimples 50 isplaced in thermal contact with a second body 52 having a correspondingplurality of receptacles or chambers 54 as shown in FIG. 6. Basicallytwo bodies, one being the laminar heating element 48 and the second body50 being a fluid-receiving surface. In another variation, the secondbody is configured into a microfluidic circuit containing reservoirs,conduits or channels. In one variation, the second body is shaped withone or more geometries and the laminar heating body is planar. Inanother variation, both the laminar heating body and the second body areshaped to define corresponding channels and reservoirs. Of course, oneor more heating elements alone or printed in series can be placed inthermal communication with a channel or any other structure or geometryand, of course, the second body may be planar.

In one variation, the laminar body is placed into a mold and integrallyformed with a second body by a process known in the art as in-molddecorating (IMD) which is commonly used in automobile interior trim andhand-held electronics. Materials that can be used for the combinedlaminate, including the second body, include plastics, polymers,thermosets, thermoplastics, metals, papers, glasses, ceramics orcomposites containing more than one of these materials. It is oftenuseful to use at lease some materials which are flexible, and therefore,can be processed in roll form, allowing continuous processing.

With in-mold decorating the laminar element serves as an insert that isembedded into a second body. The laminar element is typically pre-formedwith dimples or other geometries prior to insertion into a mold cavity.In one variation, the printed circuit pattern on the laminar element isprinted such that the desired pattern is distorted according to thegeometry to be formed such that the distorted pattern reaches a desiredpattern when a particular geometry is subsequently formed in the laminarelement. Liquid resin, such as a polycarbonate/polybutelyen terepthalateblend by the trade name GE Xenoy, is shot behind the inserted laminarelement, bonding the surface of the laminar element to the molding resinand forming a finished integral part. The result is a durable in-moldheating element that is in excellent thermal contact because it is anintegral component of the molded assembly. No dust or other particlescan come into contact with the circuit to affect it or scratch anddamage the layers. The most common resin materials for the assemblyinclude polycarbonate, SAN, Cycoloy, PVC, nylon, ABS, styrene, acrylic,polypropylene, polyethylene and polystyrene. Other processes areemployable to place the laminar element in thermal contact with a secondbody. The invention is not limited to forming a micro-well plate with anintegral heating element as shown in FIG. 7. Any geometry is within thescope of the present invention. For example, the laminar body can beformed into a micro-centrifuge tube or microfluidic circuit containingany shape channel, conduit, chamber and reservoirs. In the case of amicro-well plate, a plurality of leads are connected to the electrodesof each resistance heating element circuit individually in a“bed-of-nails” contact scheme for multiple heaters in a small area andthereby controlled separately. Alternatively, the entirety ofmicro-wells is connected with a common bus bar with one contact andcontrolled as a single unit. This construction advantageously providesintimate contact with the heating element and a short distance betweenthe heating element and the fluid to be heated. The device can be madeto have a low thermal mass resulting in low power usage. A power sourcefor applying a voltage across the heating element or elements isconnected to the electrodes. Control of the power source is carried outby an appropriately programmed controller, such as a computer,microprocessor, or microcontroller in an external instrument.

In one variation, the heating device according to the invention includesan electronics component. The electronics component may include signaldetection circuitry. The signal detection circuitry may detectelectrical fields, electrical current, temperature, conductivity,resistivity, magnetic fields, dielectric constant, chemical properties,pressure, or light, depending on the operational requirements of thedevice. The techniques utilized for detection of these properties areknown in the microfluidics and electronics art. It should be understoodthat circuitry for detecting other phenomena may also be included withinthe electronics component.

The electronics component may also include signal processing circuitry.For example, the signal processing circuitry may amplify a signal,filter a signal, convert a signal from analog to digital, and makelogical decisions based upon signal inputs. Because the possibilitiesfor signal processing are numerous, it should be understood that anytype of signal processing is anticipated for implementation in theelectronics component.

The electronics component may also provide circuitry for controlfunctions such as voltage control, current control, temperature control,clock signal generation, etc. For example, the electronics component mayconvert power incoming to the system at 15 volts into 5 volts forutilization by the electronics component. The electronics component mayalso be utilized to create certain desired signals, such as sinusoidalsignals. Flow control circuitry may be incorporated in order tomanipulate microfluidic flow control elements of various types such asvalves, pumps, and regulators. As with the detection and processingcircuitry, the possibilities for control circuitry are numerous andtherefore it should be understood that any type of control circuitry isanticipated for implementation in the electronics component.

The electronics component may also contain software or firmware that,through its operation, guides or controls the action of the circuitry.For example, the electronics component may contain programmable logicwhich allows a programmed algorithm to be executed so as to performcertain functions. These functions may include signal filtration, signalfeedback, control operations, signal interruption, and other forms ofsignal processing. The controller may be programmed to take a chamber orchambers of the micro-well plate and fluid deposited therein through anynumber of predetermined time and temperature profiles by varying theamount of power supplied to one or more laminar heating elements.

In the case where the resistive layer comprises a PTC composition, theresistive layer will be conductive at temperatures below its switchingtemperature, T_(s). Below T_(s), current flowing through the electrodespasses through a portion of the resistive element thereby heating theelement and chamber and any fluid deposited in the chamber that is inthermal contact with the element. Above T_(s), the increase inresistance is typically sufficiently high that the heating element iseffectively converted from an electrical conductor to an electricalinsulator by a relatively limited increase in temperature. When thetemperature reaches the preselected switching temperature of the PTCcomposition, the conductivity changes precipitously to a low conductivestate typically at a temperature near the characteristic crystallinemelt temperature of the crystalline organic chemical. The resistivelayer is no longer conductive in this state and the current ceases toflow causing the heating to stop. When the resistive layer cools tobelow the switching temperature, the polymer of the resistive materialbecomes crystalline and the resistive layer reverts back to a highconductivity state until it again reaches the switching temperature. Thedevice continuously cycles in this manner. A further practicalrequirement for most PTC materials is that they should continue toexhibit useful PTC behavior with T_(s) remaining substantially unchangedwhen repeatedly subjected to thermal cycling.

Accordingly, the switching temperature of the PTC material ispreselected by selecting the switching temperature of the resistivematerial and thereby to control the temperature at which the elementself-regulates such that the temperature of the chamber remains at thedesired temperature. Since, in part, ambient conditions affect thetemperature-dependent behavior of the heating element, the ambientconditions can also be adjusted such that the temperature of the chamberremains self-regulated at a desirable temperature. For example, a vacuumcan be applied to the chamber as a means for regulating ambientpressure.

The behavior of a PTC resistive layer having a switching temperatureabove 70° C. is shown in FIG. 8. Referring now to FIG. 8, with novoltage being applied to the element, the temperature of the element isapproximately 20° C. and the pressure of the chamber is at approximately760 torr. When 12 volts are applied after approximately one minute,current begins to flow through the device at approximately 12 mA and thedevice temperature increases to over 60° C. When 50 μL of water isplaced in the chamber, the chamber is cooled to approximately 45° C.With this cooling, the resistive material becomes more conductive and asa result there is a spike in current to approximately 30 mA. Theresistive layer and the droplet begins to gradually increase intemperature and the current settles to a plateau of about 27 mA afterthe spike before returning to its original baseline current of 12 mAthat was measured before the droplet was placed into the chamber. At thepoint when the current returns to the original 12 mA, the 50 μL of waterhas completely evaporated.

In an embodiment in which the thermal cycling is below the switchingtemperature as described above with respect to FIG. 8, the current, forexample, can be detected using detection circuitry of the electronicscomponent. As such, the laminar element serves as a temperature controldevice such that the current is calibrated to a correspondingtemperature under a constant voltage as shown in FIG. 8. In other words,the heater element resistance is a function of temperature so thisproperty can be used to measure temperature. Also, the electronicscomponent can used to vary the current to control temperature or measurea fluid volume. For example, the amount of power required to raise theelement to a specific temperature will be determined by how muchevaporative cooling is occurring which is a function of how much liquidis in thermal contact with the resistance material of a heating element.Measuring the differential power required to keep the element at aspecific temperature provides a measure of how much liquid is one theelement.

The heating element also functions as a sensor wherein a particularelectrical current is calibrated to correspond to a particulartemperature. This temperature can be calibrated to correspond to thetemperature of the resistance material, the temperature of the chamber,or the temperature of a particular fluid under given ambient conditions.As a sensor, the laminar element need not function as a heater. Forinstance, once the electrical current is detected and a temperatureoutput received from the electronics component, the voltage supply tothe circuit can be removed or the circuit interrupted such that theresistance material does not substantially heat the fluid volume.

Alternatively, a device combining two heating elements such that oneserves as a heater and such that another, which is electricallyconnected to the first, serves as a temperature sensor. In such anembodiment, a controllable heating element is printed within or adjacentto a region for thermal control of a fluid volume. The temperaturecontrolled region also includes a similar heating element serving as atemperature sensor for monitoring temperatures and thereby controllingthe application of current across the heater such that thermal controlof the fluid volume is carried out by varying the current supplied tothe heater to achieve the desired temperature for particular fluidvolume. A wide variety of sensors are available for determiningtemperatures may also be used. In such a system, the controller, powersource, heating element and temperature sensor may form a closed looptemperature control system for controlling the temperature of a chamberor fluid volume. In another embodiment, circuits are printed on oppositesides of the substrate. In one example, a heating element is printed onone side of the laminar layer and a sensing element is printed on theother side of the laminar layer. If current (heat) is supplied at oneside of the microfluidic device, the temperature on the other side willbe highly dependent on whether or not fluid is present.

In another embodiment, the heating element functions as a combinedtemperature sensor and heating element to provide heating andtemperature control of a fluid volume. The device is useful fortemperatures above ambient such that applied current would provide heatto raise the fluid temperature above ambient. Ambient temperature refersto the temperature surrounding the device locally and may be differentthan the temperature of the room that the device is in. The electronicscomponent detects and monitors any electrical signal, such as current,resistance or temperature through its detection circuitry and provides asignal that is calibrated and used to control the temperature of fluidvolumes within the device. In one embodiment, an electronic bridgecircuit such as a Wheatstone bridge is used. The bridge circuit monitorsthe resistance of the device, for instance, by measuring the currentthrough the device at a known voltage. The bridge circuit has a supplyof voltage coupled across two opposite terminals. The remaining twoterminals, which thereby carry the bridge output, are coupled to theinputs of an operational amplifier, for example. Two adjacent arms ofthe bridge are comprised of two resistors whose ratio determines abridge constant for balancing the bridge. Of the two remaining arms ofthe bridge circuit, one includes a third resistor and the other includesa resistive heating element having a positive temperature coefficient.When heat is conducted away from the vicinity of the heating element,the heating element cools, and its electrical resistance decreases,thereby unbalancing the bridge. Through appropriate electroniccircuitry, the condition of unbalance causes a relatively large heatingcurrent to flow through the heating element, causing the latter togenerate more heat, until the resistance of the heating element againreaches the point where the bridge is in balance. The bridge constantcan be preselected by using a variable resistor for example calibratedfor a “dialed-in” desired temperature. Also, the voltage supply can beadjusted to a desired current across the heater element resistor andhence the temperature at the heater. A controller is programmed toadjust the amount of power supplied to the heating element in dependenceupon the resistance of the heating element resistor. The signal measuredat the bridge will typically be input to a processor or microcontrollerwhich is programmed to receive and record this data. The same processorwill typically include programming for instructing delivery ofappropriate current for raising and lowering the temperature of theregion of interest. For example, a processor may be programmed to takethe interactive region through any number of predeterminedtime/temperature profiles such as thermal cycling for polymerase chainreactions (PCR) and the like. Given the small size of the device andreservoirs, cooling of an interactive region of interest will typicallyoccur through exposure to ambient temperature. The vacuum to which thereservoir is subjected may also be controlled to control the rate ofevaporation. However, additional cooling elements such as coolantsystems, Peltier coolers, water baths, heat pipes and the like may beincluded if desired. An error signal can be used to stabilize thetemperature of the heater by feedback control. Thus, the device servesas both a temperature sensor and as the heating element. The circuitdoes not directly measure temperature, but rather “sees” only theelectrical resistance of the heating element or current differential.

In some cases, it is desirable to control the temperature to within onedegree centigrade and sometimes within less than a tenth of one degreecentigrade as in microfluidic applications. In other cases, it is onlydesirable that the temperature be controlled to within about 10 degreescentigrade. The invention can accommodate all of these ranges oftemperature control depending on the design. In cases where a fine levelof temperature control is needed, other factors such as, the ambienttemperature and the rate of heat loss to ambient, are controlled. Forexample, the rate of heat loss from the device can be controlled withthe use of insulating materials surrounding the device.

The use of a PTC material advantageously prevents and/or reduces theformation of hot spots in the following manner. When hot spots begin toform on the edges of a coated electrode due to current concentration,the resistance of the resistive layer at the electrode edges increases,resulting in a reduction in current flowing to and through these hotedges with an ultimate decrease in temperature of the edges and anymaterial in contact or near the edges.

Furthermore, the heating element is suitable for fluid concentration.For example, the invention provides a device for concentrating theanalyte into a volume of elution fluid smaller than the original samplevolume. The desired analyte may comprise, e.g., organisms, cells,proteins, nucleic acid, carbohydrates, virus particles, bacteria,chemicals, or biochemicals.

As shown in FIG. 8, evaporation of a fluid volume to dryness results inthe electronics component detecting a return to a baseline, orpreselected current across the PTC material. Under conditions of ambientpressure of 760 torr, a square circuit pattern dimensioned 0.250 by0.250 inches with a trace width of 0.010 inches and a gap width of 0.020inches and a resistive layer of DuPont 7283 approximately 7 micronsthick, all printed on a PET substrate of 0.010 thickness 50 μL ofdistilled water completely evaporated after approximately 12 minutes.The current or resistance across the resistive body is calibrated forvolume measurement of a particular fluid such that at a certain readingof current, temperature, or resistance across the circuit, the fluid isknown to have been concentrated to a certain preselected. In thisvariation, a single heater element is used to concentrate a fluid volumein a chamber or reservoir of the device.

In addition to heating and concentrating fluids, the invention also isused to measure the volume of fluids. One way of measuring fluids withthe invention relies on the fact that the heat capacity of a region ofthe device is highly dependent on whether it contains fluid or air. Aresistive circuit is printed on a region of the device below a channelor reservoir and the amount of current needed to raise the temperatureof the region will be strongly dependent on the presence of fluid. Ameasurement of fluid is provided with a single resistive element printedon a film.

In another variation, multiple heating elements are configured toaccurately measure fluid volume and/or to concentrate a fluid to adesired volume. As shown in FIG. 9 a, one or more heater elements 58 areformed in a circuit that is in thermal contact with a single reservoir56. One or more heater elements 58 are formed at least along one portionof the reservoir at known distances corresponding to known fluid volumegraduations. In one variation, one or more heater elements 58 are formedalong at least a portion of the entire length L of an outer surface of areservoir or other structure formed in the laminar heating body. Forexample, in FIG. 9 a, heater elements 58 spaced at known reservoirvolume graduations are shown located on a reservoir 56 having aspherical geometry. When a volume of fluid 60 is deposited in thereservoir 56, a change in resistance of the PTC body will be detected bythe detection circuitry in those heating elements that are in thermalcommunication with the fluid. Depending on which of the one or moreheater elements 58 are so triggered or not triggered, the volume offluid 60 in the reservoir is determined. The device of FIG. 9 a can alsobe used to concentrate a volume of fluid in the reservoir 56 to adesired volume by activating one or more of the heating elements 58 toheat the chamber 56 and evaporate the fluid until a desired volume offluid is reached as indicated by a detected electrical signal such as aresistance or current reading against the known graduations.

In FIG. 9 b, there is shown one or more heater elements 62 that arespaced at or above a known graduation of reservoir 64 volume. Forexample, the known graduation above which the one or more heatingelements 62 are located is 10 μL. When a volume of fluid is deposited inthe reservoir 64, a change in resistance of the PTC body will bedetected by the detection circuitry in those heating elements that arein thermal communication with the fluid volume. Depending on which ofthe one or more heater elements 62 are so triggered or not triggered,the volume of fluid in the reservoir 64 is determined. From there, thedevice of FIG. 9 b can is used to concentrate a volume of fluid in thereservoir 64 to the desired graduation by activating the at least one ormore heating elements 62 until a detectable electrical signal such asthe current or resistance indicates that fluid is not in thermalcommunication with any of the one or more heating elements 62. Thereservoir 64 having a cone-shaped geometry is shown for exemplarypurposes only and is not intended to be limiting.

FIG. 10 a shows a top view of an embodiment of a laminar body 66 havingone or more heating elements 68 arranged in a concentric pattern andFIG. 10 b shows a perspective view of the laminar body 66 with a volume70 of fluid deposited thereon. In the case of a single heating element,the concentric circles of FIGS. 10 a and 10 b represent conductivematerial of a single heating element that is configured in a pattern ofconcentric circles such as shown in FIG. 1. In the case of more than oneheating element, the concentric circles of FIGS. 10 a and 10 b representa singular heating element arranged concentrically with at least oneother heating element wherein each heating element is individuallycontrollable. For example, each concentric circle represents separateheating elements 72, 74, 76, 78. Alternatively, concentric circles 72and 74 represent one heating element and circles 76 and 78 represent asecond heating element. Concentric circles are used by way of exampleand not intended to be limiting. Also, although the laminar body of FIG.10 is shown to be planar, the invention is not so limited and theembodiments described herein are applicable to any geometry.

When a volume 70 of fluid is placed on the one or more of the heatingelements 72, 74, 76, 78 as shown in FIG. 10 b, the resistance of the oneor more heating elements changes such that current increases across theresistive layer to heat the fluid volume. The current/temperatureresponse is similar to that shown in FIG. 8 for each of the one or moreheating elements. In the case of more than one heating element and byway of example, heating element 78 will exhibit a return to a baselinecurrent when fluid volume has evaporated such that it is not in thermalcommunication with heating element 78 at which point it may be turnedoff. As evaporation continues, and volume of fluid decreases and heatingelement 76 will exhibit a current response indicative of evaporation. Ittoo may be controlled to shut down. This concentric heating continuesuntil the fluid volume reaches a known desired volume. A coatingcomprising fluorinated urethane can be applied to the heating element toincrease the surface hydrophobicity.

Yet another variation is shown in FIG. 11 a. In this variation, one ormore heating elements are configured on a substrate surface. Forexemplary purposes, FIG. 11 a shows three independently controlledheating elements 80, 82, 84 arranged in a linear pattern on a substratesurface 86; however, the invention is not so limited and any number ofheating elements and patterns are within the scope of the invention.With a volume 88 of fluid placed on the heaters 80, 82, 84, the powerdifferential required to keep each of the resistive heaters at aspecific temperature is measured via the electronics component. Theamount of power required to raise the element to a specific temperatureis dependent on a number of factors including in part the ambienttemperature, pressure, the heat transfer coefficient of the device, thethermal conductivity of the medium surrounding the device, the movementif any of the medium and the surface area of the device. As with asingle heating element described above, an electrical signal such as thecurrent, resistance, temperature and amount of power required to raisethe element to a specific temperature will be a function of the amountof fluid that is in thermal contact with the resistance material of eachheating element. Summarily, the entire fluid volume is measured. Thus,as the fluid volume 88 evaporates and reduces in size such that it doesnot contact the side heating elements 80 and 84 as shown in FIG. 11 b, adifferential change in the power required to keep the elements at aspecific temperature, for example, can be detected. As the fluid volume88 decreases the heat load on side heaters 80 and 84 will decreasefaster than the middle heater 82 and when the fluid volume 88 does notcontact the side heaters 80, 84, the heat load will be much less anddetectable by measuring the differential power to keep the middle heater82 at a specific temperature relative to the side heaters 80, 84. Hence,the fluid volume 88 is measured and can be concentrated to a desiredvolume.

A heater according to the invention is incorporated into a microfluidiccircuit for heating a chamber, reservoir, channel or conduit. The heaterallows for highly efficient elution of the analyte from the chamber sothat a large amount of analyte may be released into a small volume ofelution fluid. The heater may also be used to facilitate capture of theanalyte. One advantage of the use of a heater in a small volumemicrochamber is that minimal energy is required to heat the chip.

Another advantage of the microfabricated chip is that it allows forrapid and direct heating of the internal attachment surfaces of thechamber. The integral nature and high thermal conductivity of thechamber walls and column structures allow for rapid heat transfer fromthe heating element directly to the attachment surfaces withoutnecessitating heating of the fluid in the chamber. This improvement inefficiency is significant in terms of the speed, precision, and accuracyof the heating, as well as in the reduction in power required for theheating. In particular, the rapid and direct heating of the internalsurfaces to which the analyte is bound greatly increases the degree andefficiency of the elution, and provides a significant improvement overprior art methods and devices. Tests have shown that recovery of nucleicacid with concentration to approximately 10 μL was approximately greaterthan 80%. Also, recovery of nucleic acid with concentration to drynessand then reconstituted with 10 μL of distilled water was approximatelygreater than 80% with the reservoir surface pre-treated with fluorinatedurethane to increase surface hydrophobicity.

In one embodiment, the laminar heating element of the present inventionis used to move microfluidic fluid volumes in a microfluidic circuitcomprising channels, conduits, chambers and the like. In thisembodiment, one or more laminar heating elements are positioned inthermal communication with a reservoir, channel, conduit, chamber orother geometry containing an air reservoir that are formed in thelaminar body or in a second body. For example, a first conduit isinterconnected to a second conduit wherein one or more heating elementsare located proximate to the first conduit. The second conduit is filledwith fluid and the first remains filled with air. When the heatingelements are activated, the heating elements expand the volume of air byheating the area and raising the temperature of the air or other fluid.The expanding air or gaseous fluid exerts pressure on the fluid volumein the second conduit to move the fluid volume through channels andchambers of the fluidic circuit. Although the term conduit is used forexemplary purposes, the invention is not so limited and any geometry canbe employed and substituted for the first and second conduits.

While the present invention has been described with reference to one ormore particular variations, those skilled in the art will recognize thatmany changes may be made thereto without departing from the spirit andscope of the present invention. Each of these embodiments and obviousvarious thereof are contemplated as falling within the spirit and scopeof the claimed invention, which is set forth in the claims.

1. A method for concentrating and measuring microfluidic volumescomprising the steps of: providing a laminar body comprising: asubstrate having a first surface and a second surface; at least oneheating element disposed on the first surface; the heating elementcomprising: a conductive layer patterned into at least two electrodes inspaced relation to each other; a resistive layer comprising a resistivematerial having a resistance temperature coefficient such thatresistance changes with temperature; the resistive layer being disposedbetween the electrodes to permit current to flow through the resistivematerial between the electrodes; and at least one fluid-receivinglocation corresponding to the location of the at least one heatingelement wherein the heating element is in thermal communication with thefluid-receiving location; placing a volume of fluid at thefluid-receiving location; providing an electronics component having atleast signal detection circuitry and control circuitry connected to theat least one heating element; applying a voltage across the at least oneheating element; obtaining at least one electrical information from atleast one of the heating elements; the electrical information being afunction of the variable resistance of the resistive material;monitoring the electrical information of the at least one heatingelement; controlling the at least one heating element based on theelectrical information calibrated to correspond to a known fluid volumeor temperature; and determining the fluid volume or temperature.
 2. Themethod of claim 1 wherein the step of monitoring the electricalinformation includes monitoring the electrical information of one ormore of the heating elements.
 3. The method of claim 1 wherein the stepof determining the volume of fluid includes calculating the fluid volumeas a function of the resistance of the at least one heating element. 4.The method of claim 1 further including the step of: positioning the atleast one heating element along a surface of the fluid-receivinglocation at locations corresponding to known graduations of one or morefluid volumes; determining the level of the fluid based on at least oneelectrical information of the at least one heating element.
 5. Themethod of claim 1 further including the step of evaporating the fluidvolume.
 6. The method of claim 1 wherein the step of monitoring anelectrical information includes monitoring the voltage or current acrossthe at least one heating element.